1. Field of the Invention:
The present invention relates generally to the production of urethane foams, and more particularly to a process for producing a urethane foam having a high density outer surface layer (which is of the so-called "urethane foam with integral skin" type) suitable for use as interior parts of an automobile, a ship, etc., or component parts of a furniture.
2. Description of the Prior Art:
In the production of urethane foams having a high density outer surface layer, it has been customarily practice to use the chlorofluorocarbon blowing agents such as CFC-11 or CFC-113 (CFC=chlorofluorocarbon) together with a catalyst such as 1,4-diazabicyclo(2,2,2)octane or dibutyltin dilaurate. Typical examples of such prior practice are disclosed in Japanese Patent Publication Nos. 55-21045 and 55-27098.
During the reactions that produce a urethane foam, the heat of reaction is conducted to a metal mold, thereby creating a temperature difference in the foaming plastic mass. This temperature difference changes the vaporization rate of physical blowing agents such as CFC-11 or CFC-113 which has been mixed with and homogeneously compatibilized in a urethane stock solution, so that an outer portion of the foaming plastic mass which contacts the mold surface forms a high density outer surface layer, while an inner portion of the foaming plastic mass which is held out of contact with the mold surface has a cellular structure. As the reaction advances, the mold cavity is filled with foams or bubbles and the blowing pressure in the mold rises (This condition is generally called as an "overpacked" state or condition having an overpack factor ranging from 4 to 7 where overpack factor is the density ratio of a molded foam to a free-blowing foam). In this instance, gases trapped in those foams which are contacting the mold surface are condensed and then absorbed or recompatibilized in the reacting plastic mass. Thus, the high density outer surface layer is thickened.
In this condition, the foaming plastic mass is forced into rapid gelation (solidification of the flowable foaming plastic mass following a rapid rise in viscosity caused by polymerization). By this gelation, the shape of the high density outer surface layer and the inner relatively low density cellular layer is set. A urethane foam having a foam core and a high density outer surface layer is thus formed by a single foam molding operation of a same urethane stock solution. The conventional manufacture of such urethane foam article having a high density outer surface layer involves the consumption of CFC-11 or CFC-113.
Since CFC-11 and CFC-113 identified above, and HCFC-123 and HCFC-141b (HCFC=hydrochlorofluorocarbon) are halogenated hydrocarbon blowing agents including fluorine and chlorine, the consumption of these halogenated hydrocarbon blowing agents causes a breakage or destruction of the ozone layer in the stratosphere which will permit a great amount of detrimental ultraviolet rays to reach the ground of the earth. The detrimental ultraviolet rays bring about various serious problems against human beings, such as the mutation, deactivation and damaging of genes and cells, an increased number of cutaneous cancer, a change of the ecosystem, and the greenhouse effect resulting from an increase in the atmospheric temperature.
With the foregoing problems in view, there have been an increased demand for a technique which enables the production of a urethane foam having a high density outer surface layer either without using CFC-11, CFC-113, HCFC-123, or HCFC141b. Such technique may include (1) a production using physical blowing agents, and (2) a substituted formation of a thick high density outer surface layer by the in-mold coating process.
The foregoing techniques are not satisfactory in terms of the production due to the necessity of an additional heating process or a vacuuming process for replacing gases in a foam with air or nitrogen, a high toxicity of a gas involved and rust created thereby, and a high reconstruction cost of the manufacturing equipment. Furthermore, the foregoing techniques are also unsatisfactory in terms of the quality of the high density outer surface layer of resultant foams.
Through a concentrated research development, the present inventors have proposed processes for preparing a urethane foam with a high density skin layer which are capable of obviate the necessity of physical blowing agents, such as CFC-11 or CFC-113 by using a certain kind of catalyst in combination with a blowing agent consisting of water. These prior proposals are described in co-pending U.S. patent application Ser. Nos. 546,840 and 546,852 both filed Jul. 2, 1990.
In the production of a steering wheel grip made of a semi-rigid urethane foam with a high density outer surface layer or skin, it has been a conventional practice to formulate a plastic liquid by using a low-melting solvent such as chlorofluorocarbons (hereinafter referred to as physical blowing agents) as a blowing agent in combination with aromatic isocyanates, polyols, a catalyst, low-molecular polyols used as a cross-linker or a chain extender, a blowing agent, and an assistant such as a pigment or a foam stabilizer. The formulated plastic liquid has an average hydroxyl number ranging from 130 to 250 mg-KOH/g and generally from 140 to 190 mg-KOH/g. When the blowing agent is substituted by water or a water absorptive resin ("water absorptive resin" is used herein to refer to water absorbed in a water absorptive resin added in the formulations), and if the amount of water or the water absorptive resin added is determined so as to generate a same amount of gas while keeping the loadings of other components remain unchanged, the average hydroxyl number of the resultant plastic liquid goes up in the range of from 250 to 300 mg-KOH/g (even when a small difference in gas trapping factor is adjusted). With this large hydroxyl number, the resultant urethane foam has a relatively large hardness and relatively small elasticity and elongation which lead to a deterioration of physical properties of the urethane foams and lower the product value of the urethane foams.
In order to prevent deterioration of the physical properties of the urethane foam, the amount of low-molecular polyols used as a crosslinker or a chain extender is reduced in which instance, however, only an insufficient amount of reaction heat is generated, slowing-down the formation of a high density outer surface layer of the urethane foam. Alternatively, if the amount of water, water absorptive resin, or a mixture thereof is reduced so as to adjust the hydroxyl number of the plastic liquid in the range of from 140 to 190 mg-KOH/g, the extent of foaming of the plastic liquid and the amount of heat developed by reaction are reduced. As a consequent, a sufficient difference in reaction between an outer surface layer and an intermediate layer of the urethane foam cannot be created, failing to form a high density outer surface layer. In the case of a steering wheel grip, the difference in diameter between the steering wheel grip and a metal insert is small (about 3-7 mm), so that an outer surface layer and an intermediate layer of the steering wheel grip cannot be distinguished from one another depending on a difference in density.
As against the physical blowing process performed by the physical blowing agents such as fluorocarbons, a recondensation caused by an increase in blowing pressure within a closed mold is not expected. In the case of a chemical blowing process using water as a blowing agent, a high density outer surface layer is formed by gelation concurring with a rapid polymerization of the plastic liquid brought to an over-packed state as the foaming proceeds. The formation of the high density outer surface layer is largely influenced by sealing properties of the mold. When the mold has inaccurately finished vent holes or parting lines, a large amount of plastic liquid may flow out from the mold through these inaccurately finished portions. A molded urethane foam, therefore, has a relatively large amount of burrs, flashes, fins or vent mushrooms. When the burrs or the like are removed, the texture of an intermediate foam layer appears at the outside surface of the urethane foam, making the foam unsightly in appearance. The product value of the urethane foam is considerably lowered, accordingly.
Furthermore, the amount of heat developed solely by reaction of the plastic liquid is insufficient to create a temperature difference large enough to form a high density outer surface layer due to a difference in blowing reaction between the outer surface layer and the intermediate layer.
More specifically, will be described the above-mentioned prior practice used for the production of the steering wheel grip made of a urethane foam and having a high density outer surface layer. The blowing agent comprises the low blowing point solvents such as fluorocarbons, and the plastic liquid has an average hydroxyl number ranging from 130 to 250 mg-KOH/g, and generally from 140 to 190 mg-KOH/g, as described above. A liquid mixture for the plastic liquid is prepared by mixing 90 parts by weight of polyol (Sumiphen-3063 manufactured by Sumitomo Bayer Urethane Co., Ltd.), a crosslinker composed of 7-10 parts by weight of 1,4-butanediol (first grade reagent) or 6-8 parts by weight of ethylene glycol (first grade reagent), a blowing agent composed of 12-15 parts by weight of CFC-11 (DF-11 manufactured by Daikin Industries, Ltd.), a catalyst composed of 0.8-1.0 parts by weight of Dabco33LV (manufactured by Nihon Nyukazai Co., Ltd.). and 0.02-0.06 parts by weight of U-100 (manufactured by Nitto Kasei Co., Ltd.), a desired amount of pigment and a foam stabilizer, if necessary. The mixture is added to an aromatic polyisocyanate in the chemical equivalent weight ratio of from 1:1 to 1:1.05 and the resultant mixture is foamed in a closed mold, thereby making a urethane foam.
When CFC-11 is substituted in full amount by water, 1.7-2.0 parts by weight of water is needed for generating a corresponding amount of gas. Such a large amount of water brings about various deficiencies including a reduction of product value of the urethane foam and a deterioration of physical properties of the urethane foam, as described above. In order to overcome these deficiencies, the amount of low-molecular polyol used as a crosslinker or a chain extender, water or the water absorptive resin used as a blowing agent, or a mixture thereof is reduced. Such a change in amount of formulations of the plastic liquid, however, arises different problems such as insufficient formation of a high density outer surface layer due to a reduction of the extent of foaming and a reduction in amount of heat developed by reaction, as well as an increased amount of leakage of plastic liquid through vent holes or parting lines caused by a retarded cure time and a slowdown of polymerization.